The force of attraction between oppositely charged particles is directly
proportional to the product of the charges on the two objects (q1 and q2) and inversely proportional to the square of the distance between the objects
(r2).

The strength of the bond between the ions of opposite charge in an ionic
compound therefore depends on the charges on the ions and the distance between
the centers of the ions when they pack to form a crystal.

An estimate of the strength of the bonds in an ionic compound can be obtained
by measuring the lattice energy of the compound, which is the energy given off when oppositely charged
ions in the gas phase come together to form a solid.

Example: The lattice energy of NaCl is the energy given off when Na+ and Cl- ions in the gas phase come together to form the lattice of alternating
Na+ and Cl- ions in the NaCl crystal shown in the figure below.

Na+(g) + Cl-(g) NaCl(s)

Ho = -787.3 kJ/mol

The lattice energies of ionic compounds are relatively large. The lattice
energy of NaCl, for example, is 787.3 kJ/mol, which is only slightly less
than the energy given off when natural gas burns.

The bond between ions of opposite charge is strongest when the ions are
small.

The lattice energies for the alkali metal halides is therefore largest for
LiF and smallest for CsI, as shown in the table below.

Lattice Energies of Alkali Metals Halides (kJ/mol)

F-

Cl-

Br-

I-

Li+

1036

853

807

757

Na+

923

787

747

704

K+

821

715

682

649

Rb+

785

689

660

630

Cs+

740

659

631

604

The ionic bond should also become stronger as the charge on the ions becomes
larger. The data in the table below show that the lattice energies for salts
of the OH- and O2- ions increase rapidly as the charge on the ion becomes larger.

When a salt, such as NaCl dissolves in water, the crystals disappear on
the macroscopic scale. On the atomic scale, the Na+ and Cl- ions in the crystal are released into solution.

H2O

NaCl(s)

Na+(aq) + Cl-(aq)

The lattice energy of a salt therefore gives a rough indication of the solubility
of the salt in water because it reflects the energy needed to separate the
positive and negative ions in a salt.

Sodium and potassium salts are soluble in water because they have relatively
small lattice energies. Magnesium and aluminum salts are often much less
soluble because it takes more energy to separate the positive and negative
ions in these salts. NaOH, for example, is very soluble in water (420 g/L),
but Mg(OH)2 dissolves in water only to the extent of 0.009 g/L, and Al(OH)3 is essentially insoluble in water.